Method and system for providing inter-frequency handoff in a telephone system

ABSTRACT

In the preferred embodiment, the transmission length of the non-full rate frame is shortened frame in order for the mobile terminal  100  to monitor other frequencies with a single receiver. Modulated symbols of non-full rate frame are transmitted with reduced repetition maintaining the sum of repeated symbol energy. Generally, for 1/n (n=2,4,8) length transmission, 1/m (m≧n) rate frame repeated m/n times and transmitted with n/m symbol energy of the full rate frame. The system may use rate limitation to generate a non-full rate frame. A position of the transmitted symbols may be randomized by a mobile specific predetermined random code to minimize interference between terminals different from each other. For intensive frequency scanning and synchronization signal transmission, non-transmitting period can be assigned for successive frames of some interval. For 1/n (n=2,4,8) length successive transmission, the starting position of the transmitted symbols are staggered by 1/n frame time in each frame. If there is no part of the frame to be staggered, the starting position is the beginning of the frame. Such implementation minimizes guard time overhead for frequency switching and resynchronization.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a telephone system, and moreparticularly, to a method for controlling an inter-frequency handoff ina code division multiple access (CDMA) cellular telephone system.

2. Background of the Related Art

Generally, a cellular telephone system includes a service area which isdivided into a plurality of cells. Each cell provides many mobileterminals with a radio communication service through a base station. Tomaintain high quality communication when the mobile terminal movesbetween cells, a handoff, which changes a communication link or channelwith one base station to another base station, must be timelyestablished. Therefore, the mobile terminal has to be able to measurethe quality of signals from other peripheral base stations duringcommunicatiom, and the base station system is provided with the qualityof signal information to timely perform the handoff. Such a hand off isreferred to as a mobile-assisted handoff.

In particular, the CDMA system can perform the communication while usingthe same frequency and varying an offset of a common pseudo-random codethat spreads bandwidth of transmitting signal. The CDMA system has afirst digital receiver, which is referred to a searcher for searching adifferent CDMA signal of the same frequency, and a second digitalreceiver for demodulating the data.

Typically, the CDMA system uses a plurality of second digital receivers,which are referred to as “RAKE” receiver, to receive the datatransmitted via multiple paths. Accordingly, the mobile terminal canmeasure the quality of a different base station signal of the samefrequency during the communication. In the CDMA method, the transmissionrate of the data can be varied in multilevel, and the overall radiocapacity within the transmission frequency can be increased by reducingan interference between different channels, which use the same frequencyas the transmission rate is reduced. For example, voice data has aninformation amount variable with the time, and half of the communicationis used for listening, i.e., receiving voice data, and the rest is usedfor speaking, i.e., transmitting voice data. If a variable ratetransmission is used, a fifty percent or more increase in capacity maybe produced

The system capacity in the cellular telephone system may be increased byreducing the size of the cell to increase the number of the cells withinthe service area. However, if the size of the cell is reduced, thehandoff is too often occurred generated when moving at a high speed.Hence, the base station has difficulty in properly processing thehandoff.

Accordingly, a hierarchical cell structure or a layered cell structureis used, where a macro-cell of a larger size is placed upon a micro-cellof smaller size which serves the terminal moving at high-speed, isplaced. In this cell structure, the handoff between the micro-cell andmacro-cell according to a moving speed of the terminal, as well as thehandoff between macro-cells, has to be provided.

In the hierarchical cell structure, the signal strength of the basestation of the macro-cell may be much larger than that of the basestation of the micro-cell, and therefore various problems, such ascocktail party effect, egg phenomenon, etc., may occur even in the CDMAsystem. To provide the hierarchical cell structure on differentfrequencies, in the CDMA system has to provide a (mobile-assisted)inter-frequency handoff.

To perform the (mobile-assisted) inter-frequency mobile-assistedhandoff, the mobile terminal has to be able to receive the base stationsignal of different frequency during the communication and therefore, ithas to be provided with an additional radio frequency RF receiver, oralternatively, the mobile terminal has to use a single RF receiver in atime-division multiplexing (TDM) method for receiving the signal of adifferent frequency.

The above-mentioned related CDMA cellular telephone system continuouslytransmits a forward signal from the base station toward the mobileterminal, the mobile terminal may have two or more RF receivers forreceiving the base station signal of different frequencies during thecommunication. However, this method is undesirable since the cost andsize of the mobile terminal increase.

For the alternate method in the CDMA system, the time period where themobile terminal breaks a part of reception signal and receives thesignal of the different frequency may be used for providing theinter-frequency handoff with a single RF receiver. However, such amethod may cause the degradation of the communication quality and thecommunication may be dropped as the required signaling data is notreceived.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to substantially obviatethe problems of the related art.

An object of the present invention is to provide a method for providinginter-frequency handoff in a code division multiple access cellulartelephone system capable of maintaining an excellent communicationquality.

Another object of the present invention is to rapidly establish aninter-frequency handoff between a mobile terminal and a peripheral basestation as the mobile terminal searches a frequency of the other basestation or transmits searched information.

A further object of the present invention is to allow inter-frequencyhandoff without using dual transceivers.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, a methodfor providing an inter-frequency handoff in a code division multipleaccess cellular telephone system comprises the steps of: forming anon-transmitting period in a transmission frame by controlling therepeat times of a transmission symbol in a frame and a position or formof a transmitting period with a variable rate characteristic of atransmission signal; controlling an energy transmission level of thesignal reconstructed through the aforesaid step in inverse proportion tothe repeat times to transmit it; and searching a base station signal ofa different frequency through a period where a signal of the frametransmitted through the aforesaid step is not transmitted, ortransmitting a signal to the base station of the different frequency, bya mobile terminal.

The present invention can be achieved in parts or in a whole by a methodof performing a handoff in a telephone system comprising the steps of:modulating data to a prescribed energy level of a frame unit bycontrolling repeat times of a symbol; inserting a searching period intothe modulated frame to reconstruct the modulated frame as a transmissionframe; and searching a frequency information of a station with thetransmission frame to perform a handoff.

The present invention can be achieved in parts or in a whole by a methodfor providing a handoff in a telephone system comprising the steps of:modulating a band into a modulated frame having a repeated frame periodby varying an energy transmission level based on a full rate frame;forming a non-transmitting period to the modulated frame andreconstructing a repeat transmission frame by controlling a number ofrepetition of the energy level; and searching frequency information of aperipheral base station with the transmission frame to perform a handoffand transmitting the searched information to a transmitter of a basestation.

The present invention can be achieved in parts or in a whole by a methodfor providing a handoff in a telephone system comprising the steps of:forming a non-transmitting period in a transmission frame by controllinga number of repetitions of a transmission symbol in a frame and at leastone of a position and form of a transmitting period with a variable ratecharacteristic of the transmission frame; controlling an energy level ofthe transmission symbol by an inverse proportion of the number ofrepetitions; and searching a base station signal of a differentfrequency during the non-transmitting period; or transmitting a signalto the base station of the different frequency, by a mobile terminal.

The present invention can be achieved in parts or in a whole by atelephone system comprising: a station that modulates data into atransmission frame having an energy level based on an inverse proportionof a number of repetitions of a transmission symbol, and at least one ofa position and form of a transmitting period with a variable ratecharacteristic of the transmission frame, the station forming anon-transmitting period based on the number of repetitions as asearching period; and a terminal that searches frequency informationduring the searching period and transmits the searched frequencyinformation to the station.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in detail with reference to thefollowing drawings in which like reference numerals refer to likeelements wherein:

FIGS. 1A to 1D are transmission timing diagrams applied to a forwardlink or channel of a variable rate CDMA system;

FIGS. 2A to 2D are transmission timing diagrams applied to a reverselink or channel of a variable rate CDMA system;

FIG. 3 is a block diagram showing a preferred embodiment of the CDMAcellular telephone system;

FIGS. 4A to 4E are transmission timing diagrams applied to a method forproviding an inter-frequency handoff in accordance with a preferredembodiment of the present invention; and

FIGS. 5A to 5G are timing diagrams showing a continuous frequencysearching period upon a control of the inter-frequency handoff inaccordance with one of the preferred embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1A to 1D and FIGS. 2A to 2D, respectively, show a timing diagramin accordance with a signal transmission of a variable rate used in acode division multiple access CDMA cellular telephone system of “IS-95”style (see U.S. Pat. No. 5,416,797). FIGS. 1A to 1D are applied to amethod of varying transmission energy, and FIGS. 2A to 2D is applied toa method varying the transmission time.

In other words, FIGS. 1A to 1D are signal transmission timing diagramsof a forward link (voice channel) transmitted from a base station to amobile terminal, and FIGS. 2A to 2D are signal transmission timingdiagrams of a reverse link (voice channel) transmitted from the mobileterminal to the base station.

A transmission frame of the voice channel transmitted from the basestation is divided into sixteen subframes, and is transmitted to themobile terminal as a variable rate of four levels, such as a full rate(FIG. 1A), a half (½) rate (FIG. 1B), a quarter (¼) rate (FIG. 1C) andan eighth (⅛) rate (FIG. 1D). Then, if the data rate is reduced to 1/n(herein, n is 2, 4 and 8), the number of bit per the frame of thetransmission data is accordingly reduced and therefore, transmissiondata may be transmitted by a repetition of n times to keep transmissiondata rate constant.

If a transmission party repeatedly transmits each symbol of transmissiondata, a reception party combines the repeated symbols to achieve timediversity. The symbol is repeatedly transmitted at the different timeand thereby, the transmission symbol can be correctly demodulatedalthough parts of the repeated symbols are corrupted. The repetition isperformed after a sequence of transmission data is all transmitted sothat each repeated symbol is transmitted with enough time distance formaximizing the effect of the time diversity.

With reference to FIGS. 2A to 2D, the transmission frame of voiceinformation transmitted from the base station is distributed into setrandom patterns in a transmission/reception period to be transmitted.For example, in case of FIG. 2A, all frames are transmitted. In case of½ rate shown in FIG. 2B, any one of two subframes is transmitted, and incase of ¼ rate of FIG. 2C, any one of four subframes is transmitted. Incase of ⅛ rate of FIG. 2D, any one of eight subframes is selected to betransmitted to the receiver of the terminal in the random pattern.

FIG. 3 is a block diagram showing a transceiver applied to the method ofcontrolling the inter-frequency handoff in the CDMA cellular telephonesystem in accordance with the present invention. With reference to FIG.3, an inter-frequency handoff transceiver in the CDMA cellular telephonesystem is comprised of a receiving portion 100 of the mobile terminaland a transmitting portion 200 of a base station.

The receiving portion 100 of the mobile terminal includes an RF receiver102 receiving a radio signal selected in the radio signals transmittedfrom the transmitting portion 200 of the base station; a digitaldemodulator 101 demodulating the radio signal to an original signal; anda frequency synthesizer 103 generating frequency so that the RF receiver102 may selectively receive any one of the radio signals.

The transmitting portion 200 of the base station includes a digitalmodulator 202 forming the transmission frame of data in a frame unit andconverting each bit to a modulation symbol; a gain control element 203controlling an energy level of the modulated transmission symbol by n/m;and an RF transmitter 201 varying and amplifying a frequency of the gaincontrolled radio signal to transmit it to an outside through an antenna.

In the preferred embodiment, the transmission length of the non-fullrate frame is shortened in order for the mobile terminal 100 to monitorother frequencies with a single receiver. Modulated symbols of non-fullrate frame are transmitted with reduced repetition maintaining the sumof repeated symbol energy. Generally, for 1/n (n=2,4,8) lengthtransmission, 1/m (m≧n) rate frame is repeated m/n times and transmittedwith n/m symbol energy of the full rate frame. A frame of other channelsexcept voice channel to a mobile terminal is not transmitted during theframe having a non-transmitting period. The system may use ratelimitation to generate a non-full rate frame.

FIG. 4A illustrates a full (n=1) transmission at ½ (m=2) rate frame withtwo (m/n=2/1) time repetition. FIGS. 4B-4D illustrate ½ transmission(n=2) for ½ (m=2) rate frame, ¼ (m=4) rate frame and ⅛ (m=8) rate frame.As shown in FIG. 4B, the ½ rate frame is transmitted without repetition(m/n=1) with the same symbol energy of the full rate frame. In FIG. 4C,the ¼ rate frame is repeated only two (m/n=4/2=2) times and transmittedwith ½ (n/m=2/4) times symbol energy x of the full rate frame. The ⅛rate frame is repeated only four (m/n=8/2) times and transmitted with ¼(n/m=2/8) times symbol energy x (FIG. 4D). FIG. 4E illustrates ¼ (n=4)transmission at ¼ (m=4) rate frame without repetition (m/n=4/4) with thesymbol energy of the full rate frame.

As shown in FIGS. 1B and 1C illustrating respective transmission timingdiagrams for the half rate and quarter rate in the conventional method,data of the half rate and the quarter rate are, respectively, repeatedin two subframe units and in four subframe units, and the transmissionsymbol energy is controlled by dividing the full rate symbol energy bythe repeat times. As shown in FIGS. 4C and 4E, the repeat times of thesubframe is not set to a result obtained by dividing full rate into subrate, and the symbol energy of the transmitting period is set to aresult obtained by dividing the full rate symbol energy into thesubframe repeat times, while reducing or removing the repeat times.

As shown in FIG. 1C, the transmission is made through the four timesrepetition with symbol energy of x/4 in the conventional method. In thepreferred embodiment, the transmission at ¼ rate is made through the twotimes repetition with symbol energy of x/2 (FIG. 4C), or is made withoutrepetition with symbol energy x (FIG. 4E) by the digital modulator 202.If the transmission is made without reduction of the repeat times of thesubframe or without the repetition as mentioned above, a period wherethe signal is not transmitted can be made within the frame.

The gain control element 203 reconstructs the modulated frame as thetransmission frame by inserting the non-transmitting period including aterminal searching period into the modulated frame. The gain controlelement 203 can be located after the digital modulator 202 as shown inFIG. 3, or into the digital modulator 202 e.g., before basebandfiltering of the digital modulator.

If the transmission signal processed in the above-mentioned method istransmitted by the RF transmitter 201, the RF receiver 102 of thereceiving portion 100 receives the radio signal. Thereafter, the digitaldemodulator 101 demodulates the radio signal into the signal, as shownin FIGS. 4C and 4E. The mobile terminal varies a frequency of thefrequency synthesizer 103 during the period where the signal is nottransmitted in the frame (hereinafter, which is referred to a frequencysearching period), and then searches the base station signal of thedifferent frequency, thereby being capable of performing theinter-frequency mobile-assisted handoff.

However, the mobile terminal does not know the time and period of afrequency searching period. This problem can be solved by sendinginformation indicating the time when the frequency searching period isgenerated from the base station to the mobile terminal. The frequencysearching period may periodically be generated in a discontinuous form,and/or may be periodically or continuously during a given period.

On the other hand, in case of FIG. 4A, since the frequency searchingperiod can not be made for the full rate, the frequency searching periodcan be inserted by limiting the encoding rate of a source such as avocoder, if needed. Substantially, the CDMA cellular telephone systemapplies a rate limitation which properly limits the encoding rate of thevocoder for inserting signaling data into the voice frame during thecommunication, and the degradation of the voice quality due to this isnegligible.

As the above-mentioned transmitting method is applied to the reverselink transmitted from the terminal to the base station, the terminal mayuse an empty period to transmit the synchronization signal of thedifferent frequency. Accordingly, the base station of the differentfrequency can previously be synchronized with the mobile terminal forthe handoff.

A starting position of the transmitted symbols may be randomized by amobile specific predetermined random code, i.e., user long code in IS-95CDMA system, to minimize interference between terminals different fromeach other. For intensive frequency scanning and synchronization signaltransmission, non-transmitting period can be assigned for successiveframes of some interval. For 1/n (n=2,4,8) length successivetransmission, the starting position of the transmitted symbols arestaggered by 1/n frame time in each frame, for instance 20 frame. Ifthere is no part of the frame to be staggered, the starting position isthe beginning of the frame. Such implementation minimizes guard timeoverhead for frequency switching and resynchronization.

As shown in FIGS. 5A to 5H illustrating the position assignment ofsuccessive frequency searching period, the frequency searching period isenlarged as the length of the frame by modularly increasing the startingpoint of the frequency searching period in the successive frame in alength unit of the transmitting period. For example, in case of thequarter rate without repetition (FIGS. 5E and 5F), the starting point ofthe frequency searching period of the present frame is the first periodin four transmitting periods, is allocated to the second transmittingperiod in the next frame. Further, the starting point of the frequencysearching period is allocated differently and randomly for respectivemobile terminals, thereby being capable of statistically distributingthe interference between the mobile terminals.

As shown in FIGS. 5A to 5G illustrating other position assignment, eachstarting position of transmission symbols during successive frequencysearching period is alternately changed to the front part and the rearpart of the frame to maximize frequency searching period. Thisassignment is suitable for the CDMA system that uses various offsets forthe frame boundary for each mobile terminals.

As discussed previously, a method for providing interfrequency handoffin the CDMA cellular telephone system of the present invention has aneffect capable of maintaining high communication quality by rapidlyestablishing an inter-frequency handoff as the mobile terminal searchesa frequency of the different base station.

The method in accordance with the preferred embodiment fully enhancesinter-frequency handoff capability of TIA/EIA/IS-95 compatible systemwith minimum changes of the physical layer, e.g., there is no need forchannel structure modifications. There is no perceptible voice qualitydegradation even in the worst case scenario that requires intensivefrequency scanning. Moreover, power control and time tracking are wellmaintained, compared to discarding of an entire frame forinter-frequency handoff.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in a method and system forproviding an inter-frequency handoff in a code division multiple accesscellular telephone system of the present invention without departingfrom the spirit or scope of the invention. Thus, it is intended that thepresent invention cover the modifications and variations of thisinvention provided they come within the scope of the appended claims andtheir equivalents.

The foregoing embodiments are merely exemplary and are not to beconstrued as limiting the present invention. The present teaching can bereadily applied to other types of apparatuses. The description of thepresent invention is intended to be illustrative, and not to limit thescope of the claims. Many alternatives, modifications, and variationswill be apparent to those skilled in the art.

What is claimed is:
 1. A method of performing a handoff in a telephonesystem, comprising: modulating data of a non-full rate frame having atleast one repeated symbol by a first base station at a prescribed energytransmission level; reducing the number of repeated symbols whilemaintaining the prescribed energy transmission level by the first basestation to generate a searching period; and searching frequencyinformation of at least one second base station by a mobile terminalduring the searching period to perform a handoff.
 2. The method of claim1, wherein said modulating step further comprises controlling an energyper modulation symbol.
 3. The method of claim 2, wherein the modulatingstep further comprises controlling a position of the symbol.
 4. Themethod of claim 3, further comprising controlling a form of atransmitting period with a variable rate characteristic of thetransmission frame.
 5. The method of claim 4, wherein said modulatingstep further comprises the step of controlling an energy per informationbit of the symbol.
 6. The method of claim 1, wherein said modulatingstep further comprises controlling a position of the symbol.
 7. Themethod of claim 1, further comprising controlling a form of atransmitting period with a variable rate characteristic of thetransmission frame.
 8. The method of claim 1, wherein the repeat time isan integer number of repetitions.
 9. The method of claim 1, wherein thetransmission frame has the same energy level as the frame of the firstlength.
 10. The method of claim 1, wherein the modulated data isnon-compressed.
 11. The method of claim 1, wherein shortening atransmission length of the frame is shortened from a first length byreducing the number of symbol repetitions and wherein the insertedsearch period reconstructs the frame to the first length.
 12. The methodof claim 1, wherein the prescribed energy transmission level ismaintained by adjusting a symbol energy of the remaining symbols. 13.The method of claim 12, wherein the symbol energy is adjusted inproportion to a number of remaining symbols.
 14. The method of claim 1,wherein the at least one repeated symbol is identical to at least oneother transmitted symbol.
 15. A method for providing a handoff in atelephone system, comprising: modulating data by a base station into anon-full rate frame having at least one repeated symbol period byvarying an energy transmission level based on a full rate frame; forminga non-transmitting period within the modulated frame at the base stationby reducing a number of repeated symbols while maintaining the energylevel of the frame; and searching frequency information of a peripheralbase station during the non-transmitting period to perform a handoff andtransmitting the searched information to a transmitter of the basestation.
 16. The method of claim 15, wherein a non-transmitting periodis inserted into the transmission frame by controlling the number ofrepeated symbols based on the transmission symbol energy.
 17. The methodof claim 15, wherein said non-transmitting period of said transmissionframe is inserted in a random pattern through a variable rate limitationmethod.
 18. The method of claim 17, wherein said random pattern is anyone of a half rate, a quarter rate, and an eighth rate.
 19. The methodof claim 15, wherein said non-transmitting period of said transmissionframe is a frequency searching period.
 20. The method of claim 19,wherein a starting point of said frequency searching period is modularlyincreased in a length unit of a transmitting period.
 21. The method ofclaim 19, wherein a starting point of said frequency searching period isalternately changed to a front part and a rear part of a frame tomaximize the frequency searching period.
 22. The method of claim 15,wherein the non-transmitting period is formed based on a form of atransmitting period with a variable rate characteristic.
 23. The methodof claim 15, wherein the number of repetitions is an integer.
 24. Amethod for providing a handoff in a telephone system, comprising:generating a non-transmitting period in a non-full rate transmissionframe of a first base station by reducing a number of repetitions of atransmission symbol in a pre-transmission frame and at least one of aposition and form of a transmitting period; controlling an energy levelof the transmission symbol at the first base station by an inverseproportion of the number of repetitions; and searching a second basestation signal of a different frequency during the non-transmittingperiod by a mobile station that has received the transmission frame. 25.The method of claim 24, wherein the energy transmission level of thetransmission symbol is determined by dividing a full rate energy levelby the number of repetitions.
 26. The method of claim 24, wherein saidnon-transmitting period of said transmission frame is a frequencysearching period.
 27. The method of claim 24, wherein the number ofrepetitions is an integer.
 28. The method of claim 24, wherein thetransmission frame has the same energy level as the pre-transmissionframe.
 29. The method of claim 24, wherein data comprising the frame islimited to non-compressed data.
 30. A telephone system, comprising: abase station configured to modulate data into a non-full ratetransmission frame having at least one repeated symbol and a prescribedtransmission energy level, said base station forming a non-transmittingsearching period by reducing a number of repetitions while maintainingthe prescribed transmission energy level; and a terminal configured tosearch frequency information during the non-transmitting searchingperiod for performing an inter-frequency handoff and to transmit thesearched frequency information to said base station.
 31. The telephonesystem of claim 30, wherein said base station comprises: a modulatorthat modulates the data to the energy level of the transmission frame; again control element that inserts the non-transmitting period into thetransmission frame; and a transmitter coupled to said gain controller totransmit the transmission frame.
 32. The telephone system of claim 30,wherein said terminal comprises: a receiver that receives thetransmission frame; a demodulator coupled to demodulate the receivedtransmission frame; and a frequency synthesizer that varies a frequencyduring the non-transmitting period and searches the frequencyinformation of a peripheral base station having a different frequency toperform a handoff.
 33. The telephone system of claim 30, wherein thenumber of repetitions is an integer.
 34. The telephone system of claim30, wherein the prescribed transmission energy is determined based on aninverse proportion of a number of repetitions of a transmission symbol,and at least one of a position and form of a transmitting period with avariable rate characteristic of the transmission frame.
 35. The methodof claim 24, wherein the mobile station transmits a signal to the secondbase station to perform the search for the second base station.